It has become common to treat a variety of medical conditions by introducing an implantable medical device partly or completely into the esophagus, trachea, colon, biliary tract, urinary tract, vascular system or other location within a human or veterinary patient. For example, many treatments of the vascular system entail the introduction of a device such as a stent, a catheter, a balloon, a wire guide, a cannula, or the like.
For certain applications, the medical device is coated with a therapeutic agent adapted to expose tissue within the body to the therapeutic agent. For many treatments, it may be desirable to employ a medical device having therapeutic agents only on one surface. Alternatively, it may be desirable to employ a medical device having different therapeutic agents on the various surfaces of the device.
For certain medical applications, a coating containing a therapeutic agent is applied to the external surface of a medical device. The medical device may be configured to bring the coating into therapeutically effective contact with the wall of a body vessel. For instance the medical device may be a radially expandable tubular stent formed by a plurality of interconnected members defining open cells extending between an external (abluminal) surface and an internal (luminal) surface. A releasable therapeutic agent may be applied to the abluminal surface of the stent for delivery to a treatment site within a body vessel. The luminal surface defines a tubular lumen extending axially from the proximal end to the distal end of the stent. Such coated stent structures are commonly deployed within a body vessel to maintain patency of a stenosis, and the therapeutic agent may be selected to mitigate or prevent restenosis of the body vessel after dilation.
Coatings have been applied to medical devices by processes such as dipping, spraying, vapor deposition, plasma polymerization, and electrodeposition. Although these processes have been used to produce satisfactory coatings, they have numerous, associated potential drawbacks. For example, it may be difficult to achieve coatings of uniform thicknesses, both on individual parts and on batches of parts. Furthermore, coating material may collect around contact points between the device and the supporting apparatus, which is referred to “webbing” of the coating. When the supporting apparatus is removed from the device, the webbed material may stick to the supporting apparatus, thereby removing some of the needed coating from the device and leaving bare areas. The webbed material may also stick to the device. In addition, coating material may collect or bridge between openings of cellular devices. Upon implantation and expansion of the device, this webbed and/or bridged material may dislodge and then fall through openings in the device. Additionally, these coating processes may require that the coated part be held during coating, which may result in defects such as bare spots where the part was held and which may thus require subsequent coating steps. Furthermore, many conventional processes require multiple coating steps or stages for the application of a second coating material, or to allow for drying between coating steps or after the final coating step.